1. Field of the Invention
This invention relates to a database system, a data-storage control system, a database server and a method of controlling operation of the database server.
2. Description of the Related Art
A database system includes a command execution unit and a search unit for searching a database in which data has been stored. If a command is applied to the command execution unit, the command execution unit applies a search command to the search unit, which responds by searching the database. Data representing the search results is supplied from the search unit to the command execution unit so that the data is output.
In a case where a database search is conducted using a search technique different from the search technique of the search unit connected to the command execution unit, another search unit must be used in place of the search unit connected to the command execution unit. However, the procedure through which commands and data are exchanged by the command execution unit and search unit is known only to the party that manufactured the command execution unit and search unit. This means that substituting another search unit for the search unit connected to the command execution unit is not conceivable. In order to conduct a database search using a search technique different from that of a search unit connected to a command execution unit, both the command execution unit and the search unit must be modified in accordance with the search technique.
If data is image data or the like, image data having a variety of expression formats, such as image data representing an image of the original size or image data representing a thumbnail image, is stored in a database. If both original-image data and thumbnail-image data have been stored in a database, applying a search command makes it possible to find the corresponding original-image data or thumbnail-image data by a search.
However, if image data having an expression format specified by a search command does not exist in a case where only one type of image data, namely the original-image data or thumbnail-image data, has been stored in the database, this image data cannot be found even though the image data having the other expression format is present. For example, when a search command for image data that is thumbnail-image data has been applied, a hit regarding original-image data will not be obtained even though original-image data corresponding to this thumbnail-image data exists. This means that the search command must be applied again upon changing the expression format. In other words, a search command for original-image data must be applied again.
Furthermore, if the result of a search is failure to obtain a hit in a particular database, the same search command must be applied to other databases.